Bastian Mei

Protection of p(+)-n-Si Photoanodes by Sputter-Deposited Ir/IrOx Thin Films.

Sputter deposition of Ir/IrOx on p(+)-n-Si without interfacial corrosion protection layers yielded photoanodes capable of efficient water oxidation (OER) in acidic media (1 M H2SO4). Stability of at least 18 h was shown by chronoamperomety at 1.23 V versus RHE (reversible hydrogen electrode) under 38.6 mW/cm(2) simulated sunlight irradiation (λ > 635 nm, AM 1.5G) and measurements with quartz crystal microbalances. Films exceeding a thickness of 4 nm were shown to be highly active though metastable due to an amorphous character. By contrast, 2 nm IrOx films were stable, enabling OER at a current density of 1 mA/cm(2) at 1.05 V vs. RHE. Further improvement by heat treatment resulted in a cathodic shift of 40 mV and enabled a current density of 10 mA/cm(2) (requirements for a 10% efficient tandem device) at 1.12 V vs. RHS under irradiation. Thus, the simple IrOx/Ir/p(+)-n-Si structures not only provide the necessary overpotential for OER at realistic device current, but also harvest ∼100 mV of free energy (voltage) which makes them among the best-performing Si-based photoanodes in low-pH media.

Iron-Treated NiO as a Highly Transparent p-Type Protection Layer for Efficient Si-Based Photoanodes

Sputter deposition of 50 nm thick NiO films on p(+)-n-Si and subsequent treatment in an Fe-containing electrolyte yielded highly transparent photoanodes capable of water oxidation (OER) in alkaline media (1 M KOH) with high efficiency and stability. The Fe treatment of NiO thin films enabled Si-based photoanode assemblies to obtain a current density of 10 mA/cm(2) (requirement for >10% efficient devices) at 1.15 V versus RHE (reversible hydrogen electrode) under red-light (38.6 mW/cm(2)) irradiation. Thus, the photoanodes were harvesting ∼80 mV of free energy (voltage), which places them among the best-performing Si-based photoanodes in alkaline media. The stability was proven by chronoamperometry at 1.3 V versus RHE for 300 h. Furthermore, measurements with electrochemical quartz crystal microbalances coupled with ICP-MS showed minor corrosion under dark operation. Extrapolation of the corrosion rate showed stability for more than 2000 days of continuous operation. Therefore, protection by Fe-treated NiO films is a promising strategy to achieve highly efficient and stable photoanodes.

Strategies to design efficient silica-supported photocatalysts for reduction of CO2

The photocatalytic reduction of CO2 by water vapor to produce light hydrocarbons was studied over a series of catalysts consisting of variable loading of Ti incorporated in TUD-1 mesoporous silica, either modified by ZnO nanoparticles or isolated Cr-sites. Unexpectedly, the performance of ZnO-Ti-TUD-1 and Cr-Ti-TUD-1 was inferior to the parent Ti-TUD-1. An explanation can be found in experiments on the photocatalytic degradation of a mixture of hydrocarbons (i.e., CH4, C2H4, C2H6, C3H6, and C3H8) under the same illumination conditions. Ti-TUD-1 exhibits the poorest activity in hydrocarbon degradation, while ZnO-Ti-TUD-1 and Cr-Ti-TUD-1 showed very significant degradation rates. This study clearly demonstrates the importance of evaluating hydrocarbon conversion over photocatalysts active in converting CO2 to hydrocarbons (in batch reactors).

The synthesis of Nb-doped TiO2 nanoparticles by spray drying: an efficient and scalable method

Nb-doped TiO2 nanoparticles were prepared by a continuous spray drying process using ammonium niobate (V) oxalate and titanium oxysulfate as water-soluble precursors. The structural and electronic properties were investigated using thermogravimetric analysis, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy. Nb was found to be mainly incorporated as Nb5+ into the TiO2 lattice resulting in a charge compensation by Ti vacancies. The characterization results indicate that Nb was homogeneously distributed within the titania lattice, and that the surface segregation of Nb, which is commonly observed for Nb-doped TiO2, was significantly less pronounced. The high homogeneity and the lower extent of surface segregation originate from the efficient atomization of homogeneous precursor solutions and the fast evaporation of the solvent in the spray drying process. As a result, the ion mobility is diminished and spheres of well-mixed precursor materials are formed. Using the continuous spray drying process followed by a controlled heat treatment, the phase composition, the crystal size and the surface area of the Nb-doped TiO2 nanoparticles are easily adjustable.

Ammonia-Annealed TiO2 as a Negative Electrode Material in Li-Ion Batteries: N Doping or Oxygen Deficiency?

Improving the chemical diffusion of Li ions in anatase TiO2 is essential to enhance its rate capability as a negative electrode for Li-ion batteries. Ammonia annealing has been used to improve the rate capability of Li4 Ti5 O12 . Similarly, ammonia annealing improves the Li-ion storage performance of anatase TiO2 in terms of the stability upon cycling and the C-rate capability. In order to distinguish whether N doping or oxygen deficiencies, both introduced upon ammonia annealing, are more relevant for the observed improvement, a systematic electrochemical study was performed. The results suggest that the creation of oxygen vacancies upon ammonia annealing is the main reason for the improvement of the stability and C-rate capability.

Optimizing the Deposition of Hydrogen Evolution Sites on Suspended Semiconductor Particles using On‐Line Photocatalytic Reforming of Aqueous Methanol Solutions

The deposition of hydrogen evolution sites on photocatalysts is a crucial step in the multistep process of synthesizing a catalyst that is active for overall photocatalytic water splitting. An alternative approach to conventional photodeposition was developed, applying the photocatalytic reforming of aqueous methanol solutions to deposit metal particles on semiconductor materials such as Ga₂O₃ and (Ga₀.₆ Zn₀.₄)(N₀.₆O₀.₄). The method allows optimizing the loading of the co-catalysts based on the stepwise addition of their precursors and the continuous online monitoring of the evolved hydrogen. Moreover, a synergetic effect between different co-catalysts can be directly established.

TiO2(B)/Anatase Composites Synthesized by Spray Drying as High Performance Negative Electrode Material in Li‐Ion Batteries

TiO2(B)/Anatase Composites Synthesized by Spray Drying as High Performance Negative Electrode Material in Li-Ion Batteries The power of spray-dried TiO2 in LIBs : TiO2(B)/anatase is synthesized by spray drying and investigated as negative electrode material in Li-ion batteries. It exhibits excellent Li-ion storage performances, especially at high charge/ discharge rates. The presence of the b phase of TiO2 improves Li-ion diffusivity. Additionally, the scalable synthesis method also allows for Nb-doping, which assists in the maintenance of the electronic conductivity as the thickness of film increases.

High-Throughput Characterization of Pt Supported on Thin Film Oxide Material Libraries Applied in the Oxygen Reduction Reaction

Thin film metal oxide material libraries were prepared by sputter deposition of nanoscale Ti/Nb precursor multilayers followed by ex situ oxidation. The metal composition was varied from 6 at.% Nb to 27 at.% Nb. Additionally, thin wedge-type layers of Pt with a nominal thickness gradient from 0 to 5 nm were sputter-deposited on top of the oxides. The materials libraries were characterized with respect to metallic film composition, oxide thickness, phases, electrical conductivity, Pt thickness, and electrochemical activity for the oxygen reduction reaction (ORR). Electrochemical investigations were carried out by cyclic voltammetry using an automated scanning droplet cell. For a nominal Pt thickness >1 nm, no significant dependence of the ORR activity on the Pt thickness or the substrate composition was observed. However, below that critical thickness, a strong decrease of the surface-normalized activity in terms of reduction currents and potentials was observed. For such thin Pt layers, the conductivity of the substrate seems to have a substantial impact on the catalytic activity. Results from X-ray photoelectron spectroscopy (XPS) measurements suggest that the critical Pt thickness coincides with the transition from a continuous Pt film into isolated particles at decreasing nominal Pt thickness. In the case of isolated Pt particles, the activity of Pt decisively depends on its ability to exchange electrons with the oxide layer, and hence, a dependence on the substrate conductivity is rationalized.

Photodeposition of copper and chromia on gallium oxide: the role of co-catalysts in photocatalytic water splitting.

Split second: The photocatalytic activity of gallium oxide (β-Ga2 O3) depends strongly on the co-catalysts CuOx and chromia, which can be efficiently deposited in a stepwise manner by photoreduction of Cu(2+) and CrO4 (2-). The water-splitting activity can be tuned by varying the Cu loading in the range 0.025-1.5 wt %, whereas the Cr loading is not affecting the rate as long as small amounts (such as 0.05 wt %) are present. Chromia is identified as highly efficient co-catalyst in the presence of CuOx : it is essential for the oxidation of water.

Tailoring Mixed-Halide, Wide-Gap Perovskites via Multistep Conversion Process

Wide-band-gap mixed-halide CH3NH3PbI3-XBrX-based solar cells have been prepared by means of a sequential spin-coating process. The spin-rate for PbI2 as well as its repetitive deposition are important in determining the cross-sectional shape and surface morphology of perovskite, and, consequently, J-V performance. A perovskite solar cell converted from PbI2 with a dense bottom layer and porous top layer achieved higher device performance than those of analogue cells with a dense PbI2 top layer. This work demonstrates a facile way to control PbI2 film configuration and morphology simply by modification of spin-coating parameters without any additional chemical or thermal post-treatment.